Sea Word had a touch tank, way back in the day, that had lots of Brazilian grammas in it, but it had to be thousands of gallons.
In most home aquariums, you're only going to be capable of supporting two Royal grammas. At the most.
I had a breeding pair in a 55, and I now have a breeding pair in my 200. Unfortunately, I lost multiple RG's while trying to form the pair I have now. IME, two RG's in the same tank will either form a pair, or one will be killed. IMHO, if you're not planing to breed them, and raise the offspring, it's best to stick with one RG per tank, because several can be killed before finding two fish that will pair up, and live together peacefully.